1. Field of the Invention
The present invention relates to a thermoplastic resin film possessing uniform physical and chemical properties in the transverse direction of the film. Furthermore, the present invention relates to a method for producing a thermoplastic resin film possessing uniform physical and chemical properties in the transverse direction by suppressing the bowing phenomenon which occurs in the process of transverse drawing.
2. Description of the Prior Art
Thermoplastic resin films, particularly, biaxially oriented polyester films, polyamide films, polyolefin films, polyvinyl resin films, polyethylene-sulfide films, etc., are used for packing as well as in various industrial and other applications.
In conventional methods for the manufacture of biaxially oriented films, the physical properties in the transverse (lateral) direction of the obtained films are generally nonuniform. This nonuniformity of films arises with particular frequency in transverse drawing processes. In transverse drawing processes, the film is drawn by holding both side margins of the film in a tenter with a clasping means such as clips and imparting tension in the transverse direction by successively shifting the clasping means. Ordinarily, this drawing process is followed by heat-setting, thereby obtaining a desired transversely drawn film. In this process, the side margins of the film are securely constrained by the clasping means, but in the central portion of the film, the effects of the clasping means are relatively small and the constraining force is accordingly weak. Consequently, when a long film is subjected to a transverse drawing by passing through a tenter, the central portion of the film is affected by the stress in the longitudinal direction generated by the transverse drawing and longitudinal movement of the film, or affected by the contraction stress generated by the heat-setting process. For example, in a case where drawing and heat-setting are performed consecutively in the same tenter, if straight lines are drawn on the surface of the film in the direction prior to entering the tenter, then these straight lines are first deformed, in the area where the drawing process commences, into a convex shape which is convex toward the direction of advance of the film, then, in the area immediately preceding the completion of the drawing process, the deformed lines are restored to recover their shape, and immediately after the completion of drawing, the lines are then deformed into a concave shape. Furthermore, at the beginning of an area of the heat-setting process, this concave deformation reaches a maximum, and thereafter these curves pass through the tenter without undergoing any further deformation, hence, the concave deformation remains after the film has emerged from the tenter. This phenomenon is known as bowing. This bowing phenomenon is the cause of the nonuniformity of physical properties in the transverse direction of the film.
Owing to the bowing phenomenon, the principal orientation axes near the two side margins of the film deviate toward the longitudinal axis considerably. That is, the angles of the orientation axes at the central and side marginal portions of the film tend to differ. Consequently, for example, the physical constants with respect to the longitudinal direction, such as the thermal shrinkage factor, thermal expansion factor, wet swelling factor, etc., are different in the central and in the side marginal portions of the film. In the applications of such films to packing or wrapping, various problems arise, e.g., the printing pitch deviates in processes such as printing, lamination or bag-making; blots appear on the surface of the film; the film curls or winds, etc. Moreover, as regards industrial applications, for example, when such films are used as the base films for floppy disks, anisotropy occurs within the surface and consequently various problems arise, such as deterioration of magnetic recording characteristics, etc.
Japanese Patent Publication No. 35-11774 discloses a method for the transverse drawing of thermoplastic resin films. The method comprises a relaxation process (i.e., substantially a cooling process) in the temperature range of 20.degree. C. to 150.degree. C. that is interposed between the transverse drawing and heat-setting processes. However, if this method is employed, the bowing phenomenon is still not reduced adequately. Japanese Laid-Open Patent Publication No. 50-73978 discloses a method for producing a stretched thermoplastic film for reducing the bowing phenomenon. In this method, a process for passing a film through a group of nip rolls is interposed between the drawing and heat-setting processes of the film. This Publication discloses that the temperature of this intermediate zone where the nip rolls are disposed should be equal to, or higher than the glass transition temperature of the thermoplastic resin film. However, at the point where the film comes into contact with the nip rolls (i.e., nip point), the rigidity of the film is low, therefore improvement with respect to the bowing phenomenon is not still adequate. Furthermore, in Japanese Patent Publication No. 63-24459, a method is proposed in which a film passes through a nip roll that is located in a middle portion of the film after the drawing process of the film, while the two side margins of the film are held so that only the central portion of the film is forcibly advanced. However, in this method, the nip roll is located in a high-temperature area within the tenter, and the roll and its peripheral devices must be cooled. Since the film temperature is high, the roll may damage the film, hence, the range of applicability of this method is restricted. Japanese Patent Publication No. 62-43856 discloses a method in which a film immediately after drawing is cooled to a temperature equal to or lower than the glass transition temperature of the film, and then multistage heat-setting is applied, while the drawing in the transverse direction is performed simultaneously with the heat-setting. However, this method comprises a complex array of processes including, in addition to a cooling process, a multistage heat-setting process and a redrawing process, and the stable regulation of the temperature within the tenter and film temperature over a long period of time is difficult. Japanese Laid-Open Patent Publication No. 62-183327 discloses a method in which longitudinal drawing of a film is performed, followed by transverse drawing in the tenter, and then heat-setting. In this procedure, the provision of a preheating zone between a transverse drawing zone and a heat-setting zone is proposed, wherein only the marginal portions of the film are preheated to a temperature equal to or higher than the glass transition temperature and equal to or lower than the heat-setting temperature. However, in this method, the temperature of the preheating zone must be regulated while maintaining a temperature gradient in the transverse direction, and therefore the regulation of the film temperature over a long period of time is difficult. Japanese Laid-Open Patent Publication No. 1-165423 discloses a method, in which a film, after transverse drawing, is cooled to a temperature equal to or lower than the transverse drawing temperature, and the film is redrawn in the transverse direction while raising the temperature in several stages. However, this method, like the method of the above-mentioned Japanese Patent Publication No. 62-43856, comprises a complex array of processes including, in addition to a cooling process, a multistage heat-setting process and a redrawing process, and the stable regulation of the temperature and film temperature within the tenter over a long period of time is difficult. Moreover, this patent Publication discloses that the length of the cooling zone should desirably be at least 1/2 the width of the film, but the reasons for this are not disclosed. Japanese Patent Publication Nos. 1-25694 and 1-25696 disclose a method, in which the direction of travel of a film is reversed at specified times when transverse drawing and heat-setting are performed. However, in this method, in order to reverse the direction of travel of the film, the film must be coiled onto a reel at a predetermined time during the process, and, since this is an off-line manufacturing method, the method involves various problems, such as limitations with respect to productivity, etc.
Thus, various procedures intended to reduce bowing have been proposed, but all of these proposals are concerned with manufacturing processes and apparatus, and hitherto no invention for this purpose has been realized through consideration of the characteristics of the film itself, such as molecular orientation, etc. In order to measure the degree of bowing on the basis of molecular orientation angles, as mentioned in Japanese Laid-Open Patent Publication Nos. 58-215318 and 61-8326, the molecular orientation angle must be measured along the entire width of the film in order to determine the degree of bowing. This is due to the fact that, in the central portion of the film, there is almost no deviation in the direction of the principal axis of molecular orientation, irrespective of the degree of bowing, therefore the assessment of the degree of bowing from the measurement of physical characteristics at an arbitrary location of the film is impossible.